1. Technical Field
The present invention relates to an electron detection device and a scanning electron microscope. More particularly, the present invention relates to an electron detection device and a scanning electron microscope capable of detecting electrons emitted from a sample with high accuracy even in a low vacuum environment.
2. Background Art
In a manufacturing process of a semiconductor device, observation of a sample and measurement of a pattern line width and the like are performed by use of an electron beam device such as an electron microscope. In the observation or the measurement of the sample by use of the electron beam device, a portion to be observed is scanned while being irradiated with an electron beam. Thereafter, an amount of electrons such as secondary electrons is converted into luminance and then displayed as an image on a display device.
A generation region of the secondary electrons emitted from the sample supplied with energy from the electron beam is spread to approximately the same extent as a diffusion region of the incident electron beam. These secondary electrons have so low energy that the generated secondary electrons are detected within about 10 nm from the sample surface by a secondary electron detector. Such a device for detecting the secondary electrons basically includes a scintillator and a photomultiplier tube, and various forms thereof have been studied.
For example, U.S. Pat. No. 6,667,478 describes a detector for detecting emitted electrons, such as secondary electrons, emitted from a sample. This detector is disposed between an electron source and an objective lens. The detector is provided with a reflector for reflecting the electrons emitted from the sample, and a meshed electrode facing the reflector. The meshed electrode is axisymmetrically divided to obtain secondary electron information from each of segments.
Moreover, Japanese Laid-Open Publication No. 63-110543 describes a device using one detector including a hole whose center is located at an optical axis through which a primary electron beam passes.
As described above, the shape of the sample surface can be displayed by detecting the secondary electrons from the sample. Particularly, use of the plurality of detectors, as in the case of the secondary electron detector disclosed in U.S. Pat. No. 6,667,478, makes it possible to obtain information on an emission direction of the secondary electrons, and also to determine whether the sample surface is flat or inclined, and the like.
As another method for dividing and detecting such secondary electrons, a detector utilizing a microchannel plate (MCP) is widely used. The microchannel plate is a plate having micron-size cylindrical electron multipliers arranged adjacent to each other. The plate has its front and back surfaces coated with metal to serve as an input-side electrode (cathode) and an output-side electrode (anode), respectively. When a voltage is applied between the electrodes, electrons entering the input-side electrode collide with a channel inner wall to emit a plurality of secondary electrons. These secondary electrons are accelerated by an electric field within the channel and repeatedly collide with the channel inner wall. Accordingly, a multiplied electron current is outputted from the output-side electrode and serves as an amplified electrical signal.
As known in the art, in observation of a sample or measurement of a pattern line width and the like by use of a scanning electron microscope, irradiation of an electron beam is performed. However, the irradiation of the electron beam causes a phenomenon of charging the sample surface. Specifically, an irradiated surface is positively or negatively charged depending on a charge difference between charged particles made incident to the sample and charged particles emitted therefrom. When such charging occurs on the sample surface, the emitted secondary electrons are accelerated or drawn back to the sample. Accordingly, efficiency in emission of the secondary electrons is changed. As a result, there arises a problem of unstable quality of the image on the sample surface.
To cope with the above problem, ozone gas or nitrogen gas may be introduced into the device, for example, to prevent charging on the sample. However, introduction of the ozone gas or the like lowers a degree of vacuum inside the device.
The microchannel plate described above is intended for use in a high vacuum environment. Thus, in a low vacuum environment, accurate measurement becomes difficult due to contamination of noise into the photomultiplier tube, or the like.
Note that, in the detector described in U.S. Pat. No. 6,667,478, respective scintillators used in four secondary electron detectors do not necessarily have the same sensitivity. Moreover, in a case where the reflectors are used, not all secondary electrons can be collected by the secondary electron detectors. As described above, the secondary electrons emitted from the sample cannot be collected with certainty. Thus, there may occur an inconvenience that the sample surface cannot be accurately reproduced.